The actual thickness of a mineralised zone, as opposed to the longer distance a drill hole travels through it at an angle.
The actual thickness of a mineralized zone measured perpendicular to its orientation. Drill holes rarely intersect mineralization at perfect right angles, so true width calculations are necessary to estimate tonnage accurately.
Downhole length always overstates thickness unless the hole is perpendicular to the zone. A company quoting downhole length without disclosing true width is flattering its own results, sometimes by a factor of two or more.
Wording such as "true width is estimated to be 60-70% of downhole length" is doing real work in a release. Read it, then mentally rescale the headline.
108 terms have their own page: the definition, why it matters to an investor, where it shows up in practice, and the mistake people make with it.